Synthesis of series of transition metal-ligand (TM-L), μ2-O-TM, μ3-O-TM-L borates: Characterization and oxygen reduction reaction (ORR) activity property exploration

A series of borates 1–6, have been synthesized under mild conditions, single crystal X-ray diffraction indicates that they possess three types of metal complex configurations in borate crystalline, exemplified as TM-L for 3 and 4, μ₂-O-TM for 2, μ₃-O-TM for 1, 5 and 6. 5 and 6 exhibit both four-coordinated and six-coordinated metal ions in one single crystal lattice. 1 to 6 are further characterized by powder X-ray diffraction, elemental analysis, IR spectrum and UV–vis spectroscopy. In addition, the ORR activities of them are performed. Among them, cobalt borate-based catalyst exhibits the best catalytic performance, with a half-wave potential of 0.82 V. Furthermore, the relationship between the ORR properties of TM complex borates and their framework densities was explored.